In the past, there have been proposed LED lighting devices designed to convert an AC voltage supplied from an AC power source to a DC voltage and apply the resultant DC voltage to a light source constituted by light emitting diodes (LEDs) to light up the light source (see e.g., JP 2012-147507 A).
FIG. 8 shows a diagram illustrating a circuit configuration of a prior LED lighting device. This prior LED lighting device has main components including a rectification circuit 1, a smoothing circuit 2, a step-down chopper circuit 3, a drive circuit 4, a switching current detector 5, and a load current detector 6. The LED lighting device uses an AC power source E1 as its input power source, and performs feedback control of a load current which flows through a light source 8 constituted by a plurality ofity of light emitting diodes, so as to light up the light source 8 at a desired lighting level. The following explanation is made to a configuration of the prior LED lighting device.
The rectification circuit 1 is a diode bridge constituted by a plurality of diodes not shown, and is connected between output terminals of the AC voltage source E1. Further, the rectification circuit 1 performs full-wave rectification or half-wave rectification on an AC voltage Vac inputted from the AC voltage source E1 and provides a resultant voltage to the smoothing circuit 2 connected to an output side of the rectification circuit 1.
The smoothing circuit 2 is constituted by a smoothing capacitor C21, and is connected between output terminals of the rectification circuit 1. The smoothing circuit 2 generates a DC voltage Vdc by smoothing a rectified voltage supplied from the rectification circuit 1 and provides the generated DC voltage Vdc to the step-down chopper circuit 3 connected to the output side of the smoothing circuit 2.
The step-down chopper circuit 3 is constituted by a switching device Q31, an inductor L31, a capacitor C31, and a diode D31. Connected in parallel with the capacitor C21 is a series circuit of the switching device Q31, a resistor R51 constituting the switching current detector 5, the inductor L31, and the capacitor C31 for smoothing. Further, the diode D31 for current regeneration is connected in parallel with a series circuit of the inductor L31 and the capacitor C31. The switching device Q31 is turned on and off alternately by the drive circuit 4. Additionally, connected in parallel with the capacitor C31 is a series circuit of the light source 8 constituted by the plurality of light emitting diodes Ld81 connected in series with each other, and a resistor R61 constituting the load current detector 6.
When the drive circuit 4 turns on and off the switching device Q31 alternately, the step-down chopper circuit 3 with the above configuration lowers the DC voltage Vdc and applies the lowered DC voltage Vdc across the light source 8. Consequently, the load current (hereinafter, referred to as an LED current Ild) which is a DC current flows through the light source 8, and each of the light emitting diodes Ld81 constituting the light source 8 lights up.
The switching current detector 5 is constituted by the resistor R51 connected in series with the switching device Q31. The switching current detector 5 measures a current flowing through the switching device Q31 (hereinafter referred to as “switching current Isw”). The switching current detector 5 provides a magnitude of a voltage across the resistor R51 as a detection value indicative of the switching current Isw to the drive circuit 4.
The load current detector 6 is constituted by the resistor R61 connected in series with the light source 8. The load current detector 6 measures the LED current Ild flowing through the light source 8. The load current detector 6 provides a magnitude of a voltage across the resistor R61 as a detection value indicative of the LED current Ild to the drive circuit 4.
The drive circuit 4 turns on and off the switching device Q31 based on the detection values provided from the switching current detector 5 and the load current detector 6 such that the LED current Ild is equal to a predetermined desired value. How to turn on and off the switching device Q31 by the drive circuit 4 is described with reference to FIG. 9 (a).
The drive circuit 4 performs on/off control of turning on the switching device Q31 at a predetermined period T1 and turning off the switching device Q31 when the detection value of the switching current detector 5 becomes not less than a turning-off threshold Th. This turning-off threshold Th is selected based on the detection value of the load current detector 6. Note that, a state in which the LED current Ild is equal to the predetermined desired value is defined as a normal state. In FIG. 9 (a), the turning-off threshold Th is Th1. To adjust the obtained detection value (LED current Ild) of the load current detector 6 to the desired value, the drive circuit 4 performs feedback control by adjusting a turned-on period Ton by means of varying the turning-off threshold Th based on the detection value of the load current detector 6. Note that, the turned-on period Ton in the normal state is represented by Ton1, and the turned-off period Toff in the normal state is represented by Toff1. For example, with regard to the IPD (trade name “MIP9E0X”) available from Panasonic Corporation, the drive circuit 4 and the switching device Q31 are provided in the same package. This IPD performs the aforementioned on/off control on the switching device Q31 to keep constant a voltage of a terminal receiving the detection value of the load current detector 6.
The aforementioned prior LED lighting device is likely to be in an abnormal state in which the drive circuit 4 fails to obtain the detection value from the load current detector 6 due to short circuiting and/or an open fault of electronic parts and/or the drive circuit 4 and fails to perform the feedback control. For example, with regard to the drive circuit 4, when a terminal which receives the detection value from the load current detector 6 is grounded, the drive circuit 4 determines that the LED current Ild is insufficient. Hence, the drive circuit 4 increases the turning-off threshold Th to Th2 (>Th1) defining the upper limit. Also in a case where the drive circuit 4 is constituted by the IPD available from Panasonic Corporation, when a terminal which receives the detection value from the load current detector 6 is grounded, the IPD determines that the LED current Ild is insufficient. Additionally, also when a terminal which transfers the detection value from the load current detector 6 to a control circuit of an inside MOSFET is directly connected to a surrounding part having a reference potential and is short-circuited, the IPD determines that the LED current Ild is insufficient.
When such an abnormal state occurs, as shown in FIG. 9 (b), the turning-off threshold Th is increased up to Th2 defining the upper limit. Hence, since the time necessary for the detection value of the switching current detector 5 to reach the turning-off threshold Th2 is increased, the turned-on period of the switching device Q31 is equal to Ton3 (>Ton1) which is longer than that in the normal state. Such an increase in the turned-on period of the switching device Q31 causes an increase in the LED current Ild.
As shown in FIG. 10, when the LED current Ild in the normal state is Ild1, the LED current Ild in the abnormality is Ild4 greater than Ild2 defining the rated upper limit. The rated upper limit Ild2 is decided in consideration of thermal stress on lighting fixtures or surrounding parts due to heat generated in the light source 8. When the LED current Ild exceeds the rated upper limit Ild2, a flow of an excess current through the light source 8 is likely to cause heat in the light source 8 and such heat may cause thermal stress on the lighting fixtures and the surrounding parts. Consequently, safety may become insufficient.
Additionally, in the prior LED lighting device, when the abnormal state causing an increase in the LED current Ild occurs, power supply to the light source 8 is terminated by means of opening a power supply path by melting a fuse. However, when the power supply path is opened, the light source 8 fails to light up. Therefore, this is inconvenient for a user.
Further, a device for sensing the heat of the driver circuit 4 may be provided. In this case, while the temperature of the drive circuit 4 exceeds a predetermined temperature, the switching device Q31 may be kept turned off. According to this method, it is possible to suppress the excess current supplied to the light source 8. However, since an LED lighting fixture for holding the LED lighting device has higher heat dissipation than that of a discharge lamp lighting device, the LED lighting fixture becomes in temperature equilibrium before the temperature of the drive circuit 4 reaches the predetermined temperature. Consequently, it may be impossible to suppress the excess current flowing through the light source 8. Moreover, since the control of the drive circuit 4 become complex, the production cost may be increased.